Process and an oven for the baking of enamels on articles of ferrous metal



Feb. 14, 1961 F. V. F. HERMANS 2,9 1,753 PROCESS AND AN OVEN FOR THEBAKING OF ENAMELS ON ARTICLES OF FERROUS METAL Filed July 1, 1955 2Sheets-Sheet 1 2 v 40 J 6 9 41 2 5 I Q 9 42 I 5 F. V. F. HEghgANSARTICLES OF FERRO Feb. 14, 1961 PROCESS AND AN OVEN FOR TH AKING OFENAMEL US METAL Filed July 1 1955 2 Sheets-'Sheet2 United States Patefit O F PROCESS AND AN OVEN FOR THE BAKING OF ENAMELS N ARTICLES 0FFERROUS METAL Fernand Victor Francois Hermans, 16 Rue Papeukasteel,Uccle, Belgium The present invention relates to a process for stovingenamels on articles of ferrous metal lying in a gaseous atmospherecontaining combustion gases. 7 It is known that articles of ferrousmetal which are raised to a temperature of several hundred degrees inthe presence of air are oxidised rather rapidly if they are, at the sametime, in contact with water vapour. This is the reason why it has beenproposed to enamel articles of sheet-iron or sheet-steel or cast-ironarticles either in electric ovens or in ovens heated by gas, oil or coalbut containing a mufile in which the articles to be enamelled areprotected from the combustion gases which heat the muflie externally andwhich contain water vapour.

Heating in electric ovens is more expensive than heating muffle ovens bygas. However, the latter ovens have the drawback of being of arelatively expensive construction, of necessitating periodic renewal ofthe muflies and of having a relatively high calorific inertia. All theseovens also have the drawback that the articles being enamelled areheated by radiation from bodies, the temperature of which is necessarilyhigher than the temperature required for baking the enamel, so that, ifthe articles being enamelled are accidentally left in the oven for toolong a time, defects, such as blisters or changes of colours, aresometimes produced in the enamel. In addition these ovens do not allowthick articles and thin articles to be enamelled simultaneously, becausethe thin articles become superheated at the moment at which the otherarticles only reach the melting point of the enamel.

There are also ovens for enamelling articles of ferrous metal, in whichthese articles are heated by radiation from a base-plate, below whichcombustion gases pass before entering the oven and quietly lick its sidewalls up to openings made in its roof in order to enable them to escapethrough a chimney. In these ovens, the combustion gases do not come intodirect contact with the articles being enamelled, notwithstanding thefact that there is no Wall separating them from the combustion gaseswhich lick the side walls of the oven. Moreover, the gaseous atmosphere,which is in direct contact with the ferrous articles being enamelled,undergoes hardly any movement because it is not influenced by the fiowof the combustion gases along the side walls. These ovens have thedrawback that their base-plate is heated much more strongly than theirside walls and that, consequently, the articles being enamelled, whichare heated by radiation, are appreciably hotter on the side opposite thebase-plate than elsewhere. The result of this is that the enamellingthat is produced there is very often irregular, although the defects dueto oxidation of the metal are generally less marked there owing to thesmaller period of heating which is due to the employment of a very hotbase-plate.

The object of the present invention is a process for enamelling ferrousarticles, whereby any trace of oxidation of the metal is obviated almostwith certainty.

2,971,753 Patented Feb. 1.4, 196;

For this purpose, in the process according to the iiivention thecombustion gases are made to travel, on contact with the articles beingenamelled, at a minimum speed of one metre per second, preferably atabout four metres per second.

Tests have shown that, when such a speed of the combustion gases wasused on contact with the articles being heated up to the melting pointof the enamel, no oxidation of the sheet or cast metal carrying theenamel to be vitrified was produced, notwithstanding the water vapourcontained in these gases and licking the articles.

A similar elfect is not obtained in ovens for drying articles beingenamelled, in which drying gases at a relatively low temperature pass,at a low speed, in contact with the articles being dried which remain inthe oven often for two or three hours. In these drying ovens, the dryinggases, which sometimes contain a small quantity of combustion gases, areat a temperature which generally does not reach 70 C. because theworkmen should be able to enter the oven. The object of the low speed oftravel of the drying gases is only to prevent the gases from becomingsaturated with water and, consequently, from becoming incapable ofremoving the large quantity of water which accompanies the enamel whenthe latter is applied to the articles to be enamelled.

If the articles coated with enamel were dried for a few hours in anatmosphere saturated with moisture, they would, even after the enamel isbaked in a mufiie, have the usual defects of imperfectly dried articles.

In contradistinction, when the articles have been prefectly dried, forexample by a known method, the enamel can be baked, even at 800 C., bypassing, in contact therewith, combustion gases which contain a largequantity of water, on condition that the speed of those gases is, atleast, one meter per second, preferably about four metres per second.

This result, which appears to be in contradiction of the hithertoaccepted ideas concerning the harmfulness of water vapour contained inthe combustion gases, can be explained by the fact that, at such aspeed, the combustion gases remove the thin layer of water vapour which,during the heating of the article being enamelled, is formed from theenamel covering the article being enamelled and remains adhering to themetal. The result of the removal of this layer of water vapour, whichcovers the articles, is also, in the case in which the combustion gasescontain sulphur compounds, to reduce the corrosive action of thesecompounds on the enamel.

According to the invention, the speed of the combustion gases on contactwith the articles being enamelled is such that, even with the watervapour present in these gases, the moisture, which adheres to the metaland which is formed as the result of the heating of the enamel toseveral hundred degrees, is carried off by these gases. The water vapourcontained in the latter can retard the removal of this moisture onlyslightly and not sufficiently for it to be possible for the metal tooxidise on contact with air before the enamel melts.

The speed of forced travel of the combustion gases on contact with thearticles being enamelled,'which is produced in the process according tothe invention, is much greater than that of the slight current of airwhich is, at times, admitted into mufiie ovens by the effect of naturaldraught, with a view to reducing the oxidation of the metal. In thelatter case, the slow renewal of the atmosphere of the mufiie has onlythe effect of allowing an easier removal of the moisture derived fromthe water of impregnation or from the water of constitution of theenamel owing to a lower content of water vapour in the air in contactwith the articles of ferrous metal being enamelled in the mufiie.

A forced travel of air on contact with glass articles being 1 glass.

enamelled in a tunnel oven has already been proposed in he preheatingZAO'IIG Of these aTtiCls'. In that C353, 1101' air comes from thecooling zone of the oven and is sent over the articles in the preheatingzones so as to lead away the solvents and the enamel vapours which areevolved and'to prevent them settling on'colderarticles inthe courseoff-being heated. It has never been stated whether the speed of this hotair was, on Contact with the article's,

sufficient to remove moisture that may be formed.

It is to be noted, in additiomtha't, in the case of glass ar 'ti'cle'sbeing enemalled, one need not fear the oxidation of the articles in thepresence 6f water vapour arid'that,

es" equently, even if moisture is formed during the preheating and isled away by the air at the same time as the solvents and the enamelvapours, the effect of this rernbval of the moisture is notto preventoxidation.

It is the same in the part of these ovens where the baking of the enamelis carried out, without a muffie, in {the presence ofcombustion gases.in this part, not only isthere no oxidation to be prevented but nospecial device hasbeen proposed for sending the combustion gases overthe articles being enamelled. Moreover, the speed of these gases isnormally much lower than that of the gases used for heating sheet-ironor cast-iron articles being enamelled. As a matter of fact, the enamelsfor enamel- ,Iing these articles have, because they have a coefiicientof expansion that is nearly that of iron or cast iron, a melting pointwhich is much higher than that of the enamels which can be applied toglass. The enamels generally employed for the treatment of ferriousarticles are enamels with a titanium base, the melting point of whichisabout 7 56 C. for those thatare to be applied to sheet ironand about850 C. for those that are to be applied to cast iron. Previously, themelting point of the enamels which were available for this purpose wasstill higher. In the enamelling of glass, the enamels used should have amelting point that is lower than the softening point of the That is thereason why only enamels, the melting point of which generally does notexceed 550 C., are

employed. The result of this is that, in the enamelling of glass, thetemperature of the heating gases is considerably lower than that of thegases for heating articles of ,ferrous metal and, consequently, thevolume and'the speed ofthese gases are considerably less.

It is to be noted that'the drying of articles of ferrous metals with theaid of air, which is displaced at 'a minimum speed of one metre persecond, has already been carried out with a view of reducing theoxidation of these articles during their drying. However, in that case,much lower temperatures were to be employed than those to which thearticles being enamelled are raised in an enamelling oven. At thesetemperatures and'in the absence of enamel, the'moisture,'derived fromthe water of constitution of the enamel, has never been removed fromthese articles.

The travel of the combustion gases on contact with the ferrous articlesbeing enamelled'may obviously be produced in various ways.

One of these ways consists in employing, in the oven, jets of gasleaving the burnerswhich are used for the heating. For this purpose, itis sufiicient to'direct the burners so that the atmosphere in the ovenisset in motion on contact with the articles. The speed of the jets mayeasily be obtained by acting on theoutlet section of the burners and onthe supply pressure.

Another object ofthe invention is an oven for carrying 'outthe processaccordingtothe invention.

Inone embodiment of the invention, an oven for the discontinuousenamelling of articles comprises gas bumers which open laterallyand arearranged in'sucha manner'that the jets of combustion-gases whichleave-them meet halfwayacross the width.

In the case of a'tunnel oven having a-p'rehe'atingzone in which thearticles already enamelled travel towardsthe outlet; giving; up-a partoi'their heat to the artic-lesbeing 16 of the oven.

4 i enamelled which are advancing towards a baking zone, this oven"comprises at least one fan, which ensures a transverse travel of thegaseous atmosphere on contact with the articles in the preheating zone,and gas burners which ensure the transverse passage of the gaseousatmosphere on contact with the articles in the baking zone.

Other features and details of the invention will appear in the course ofthe description of the drawings accompanying' the present specification,which represent diagrammatically and-only by way of example threeconstructions of an oven according to the invention.

Fig. 1 represents diagrammatically a vertical crosssection through aplane represented by the line 1-1 in Fig. 2 of an oven according to theinvention for the intermittent baking of enamels.

Fig. 2 is a vertical longitudinal sectioncorresponding to the staggeredline IIII of Fig. 1.

Fig. 3 represents diagrammatically, in plan, two'par-ts of a U-shap'edtunnel oven according to the invention, in which the articles beingenai'nelled are carried during their displacement in the oven.

- Figures 4 and 5 are, on a larger scale, cross-section'sof "the ovenshown in Figure 3 which are denoted respectively by the lines IV-'1V andV-V of Figure 3.

Figures 6 and 7 are cross-sections, which correspond respectively tothose of Figures 4 and 5, in a tunnel oven according to the invention,in which the articles are suspended during their displacement in theoven.

In these difierent figures, the same reference numerals denote'identicalelements.

The discontinuous oven represented in Figures 1 and 2 comprise gasburners 2 which are fed withgas-'and-air through pipes denotedrespectively by 3 and '4. These burners open laterally into the ovennear the base-plate and send upwards, along the side walls 5, the jetsof combustion gases formed at the outlets. These jets meet one anotherat about the middle of the width of the oven. The cur-rents ofcombustion gases are compelled to descend in the middle part of the oventowards the articles being enamelled which are carried by a grate 6.

The change in the orientation of the gaseous currents which meetoneanother is facilitated by the presence of "a deflector '7 at halfwayacross the width of the oven along its roof '8.

A part of the gases which lick the articles being en'a'nielled is ledaway by the jets leaving the bhrr'ier's, owing'tothefa'ct that openings9 are made in'two lateral supports 10 and 11 which carry the grate 6.The latter is also supported by a middle wall 12 which, at the'sainetime, assists in the division of'the gaseous current;

The oven' represented also comprises'two flues lii' which are separatedfrom the baking chamber by walls-14.

However the latter leave a passagebetween the baking chamber and thefines 13 at one of their ends. This passage is formed by means oforifices 15 near the-door V The lines 13 also communicate with conduits17 which are connected to the chimney (not shown) and with the suctionside of a fan 18. The

latter ensures a forced travel of the gaseous atmosphere on contact withthe articles being enamelled in the longitudinaldirection of the oven,that is'to-say in the direction perpendicular to the'plane determined bythe jets issuing from the burners. The fan 18 therefore also ensuresap'artial recirculation of the combustion gases coming into contact-Withthe articles being enamelled. The longitudinal current of combustiongases,which is produced by the fan 18, isdis'tributed both below andabove the grate '6.

Tests have' shown'that, when the speed of the combustion gases, oncontact with'theferrous'articlesbeing enamelled; re'aehes aminimum-value of one-metre per second, appreciable oxidation ofthese'articles no longer ture which is formed from the layer of enamelbetween the instant at which the articles begin to be heated in the ovenand that at which the enamel is melted. Preferably, the speed of thecombustion gases on contact with the articles being enamelled is aboutfour metres per second.

It can be seen that, in the process for baking enamels according to theinvention, the movement of the combustion gases on contact with thearticles being enamelled may be produced solely with the aid of gas jetsissuing from the burners but, in certain cases, the arrangement of thearticles in the oven with a view to filling it as much as possible issuch that transverse currents cannot be obtained conveniently. In thatcase, it is useful to employ a fan for producing the movement of thegaseous atmosphere in contact with the articles in the direction whichis suitable for all the faces of the latter to be capable of beinglicked by the gases at a minimum speed of one metre per second.

It is to be noted that the employment of burners, which produce atransverse travel, and of a fan, which ensures a longitudinal travel, isparticularly advantageous, because all the faces of the ferrous articlesbeing enamelled can thus be licked by the gaseous atmosphere at a speedwhich is sufiicient for removing the water vapour which is formed fromthe enamel during the heating of the articles. If it is feared that thisspeed will be too great when the enamel has melted, it can easily bereduced by stopping the fan.

The tunnel oven represented by Figures 3 to 5 comprises, in the knownmanner, a preheating zone 19, in which the articles 20 already enamelledtravel towards the outlet and give up a part of their heat to thearticles 21 being enamelled which are advancing towards a baking zone22.

In the preheating zone 19, at least one fan 23 has been provided, whichensures a transverse travel of the gaseous atmosphere on contact withthe articles 20 and 21. The speed of travel of this atmosphere is, atleast, one metre per second in order that the moisture formed by thesearticles during their heating should be removed immediately by themoving atmosphere.

In the baking zone 22, the articles being enamelled are licked by thecombustion gases which, on their contacting these articles, travel at aspeed which is also at least one metre per second, owing to thearrangement of the burners 2.

In the oven represented by Figures 3 to 5, the articles 20 and 21 arecarried and are displaced in preheating and baking zones which are widerthan their height. In this case, the burners 2 are arranged laterallyabout halfway up. In addition, the burners which are situated on oneside of the oven are so oriented that the jets which issue therefrom aredirected upwards whilst the burners which are situated on the other sideof the oven are so oriented that the jets which issue therefrom aredirected downwards. This arrangement of the burners assists the travelof the gaseous atmosphere, on contact with the articles being enamelled,at the speed required for re moving the moisture which is formed fromthe enamel during the heating of these articles.

The oven represented in Figures 6 and 7 is also a tunnel oven, but thearticles being enamelled are suspended therein. height than the Widthbecause of the dimensions of the suspended articles being enamelled. Inthis case, a fan 23 also ensures a forced travel of the air in thepreheating zone 19 on contact with the articles 21 being enamelled. Inthe baking zone 22, some of the burners 2 are arranged on one side nearthe base-plate, so that their jets are directed upwards, and the othersare arranged on the other side near the roof so that their jets aredirected downwards.

It is obvious that the invention is not exclusively limited to theembodiments represented and that many modifications may be made in theshape, arrangement and constitution of certain of the elements employedon carrying out the invention, it these modifications are not incontradiction with the subject matter of each of the following claims.

What I claim is:

1. A process for removing combined water from porcelain enamels and forvitrifying porcelain enameling material on articles of ferrous metal byfusion of the enameling material in an oven heated by combustion gases,comprising introducing said combustion gases into said oven in contactwith the enamel-coated articles, circulating the combustion gases at aneffective speed by applying a positive pressure to said gases duringcontact thereof with the enameling material being melted, removing fromsaid oven the combustion gases as they are being circulated, andremoving from said oven the combined water contained in the enamelingmaterial during the heating of the latter preceding their fusion as thecirculated gases are removed from said oven, thereby preventinginjurious oxidation of the metal by combined water originally containedin the enameling material.

2. A process according to claim 1 wherein the speed of said gases is atleast one meter per second.

3. A process according to claim 1 wherein the speed of said gases is atleast four meters per second.

References Cited in the file of this patent UNITED STATES PATENTS1,819,772 Darrah Aug. 18, 1931 2,010,295 Drefiein Aug. 6, 1935 2,181,928Vaughman Dec. 5, 1939 2,308,902 Weller Jan. 19, 1943 2,434,491 Elder etal Jan. 13, 1948 2,480,374 Larman Aug. 30, 1949 2,492,682 Carpenter Dec.27, 1949 2,517,024 Prescott et a1 Aug. 1, 1950 2,676,007 Davis Apr. 20,1954 2,676,008 Munker Apr. 20, 1954 In this oven, the baking zone is ofgreater

